Alternative methods for fabrication of substrates and heterostructures made of silicon compounds and alloys

ABSTRACT

The present invention relates to alternative methods for the production of crystalline silicon compounds and/or alloys such as silicon carbide layers and substrates. In one embodiment, a method of the present invention comprises heating a porous silicon deposition surface of a porous silicon substrate to a temperature operable for epitaxial deposition of at least one atom or molecule, contacting the porous silicon deposition surface with a reactive gas mixture comprising at least one chemical species comprising a group IV element and at least one silicon chemical species, and depositing a silicon-group IV element layer on the porous silicon deposition surface. In another embodiment, the chemical species comprising a group IV element can be replaced with a transition metal species to form a silicon silicide layer.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/074,315, filed Mar. 7, 2005, now U.S. Pat. No. 7,718,469,which claims priority to U.S. Patent Application Ser. No. 60/550,276,filed Mar. 5, 2004, which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The present invention relates to an improved method of fabrication ofsubstrates and layers made of silicon (Si) compounds and alloys throughvapor-phase heteroepitaxy as well as alloying and conversion of poroussilicon (Si) seeds.

BACKGROUND OF THE INVENTION

Silicon is the most perfected crystalline material among knownsemiconductors. The abundance of silicon and the capability to fabricatesingle crystalline silicon wafers as large as 12″ have led to economicalproduction and domestication of ultra-large scale integrated (ULSI)circuits and devices used in almost every aspect of our daily life.However, silicon can not meet the demands of high power, high speed,high temperature, and chemically inert devices. In addition, theindirect bandgap of silicon makes silicon an extremely inefficient lightemitter.

Integration of other semiconductors on silicon can alleviate some of theproblems associated with silicon. An obstacle to integration of othersemiconductors on silicon is the large lattice-mismatch of Si compoundsand alloys with single crystal Si, which makes it difficult to prepareelectronic-grade semiconductors on Si. Silicon compounds such as siliconcarbide, silicon silicides, (e.g. chromium silicide, nickel silicide,etc.), silicon germanium (Si_(x)Ge_(1-x)), silicon tin (Si_(x)Sn_(1-x)),and their ternary alloys are highly sought for wind range ofapplications. Economical preparation of electronic grades of thesematerials as layers on Si or as substrates would trigger new revolutionsin the microelectronic/nanoelectronic industry.

SUMMARY OF THE INVENTION

The present invention comprises methods of manufacturing crystallinesilicon compound layers and substrates. In one embodiment, a method forthe production of a crystalline silicon compound layer comprises heatinga porous silicon deposition surface of a porous silicon substrate to atemperature operable for epitaxial deposition of at least one atom ormolecule; contacting the porous silicon deposition surface with areactive gas mixture comprising at least one chemical species comprisinga group IV element, and at least one silicon chemical species; anddepositing a silicon-group IV element layer on the porous silicondeposition surface.

In another embodiment of the present invention, a method for theproduction of a silicon compound substrate comprises heating a poroussilicon deposition surface of a porous silicon substrate to atemperature operable for epitaxial deposition of at least one atom ormolecule; contacting the porous silicon deposition surface with areactive gas mixture comprising at least one chemical species comprisinga group IV element and at least one silicon chemical species; depositinga silicon-group IV element layer on the porous silicon depositionsurface; and separating the silicon-group IV element layer from theporous silicon substrate.

In another embodiment, a method for the production of a crystallinesilicon silicide layer comprises heating a porous silicon depositionsurface of a porous silicon substrate to a temperature operable forepitaxial deposition of at least one atom or molecule; contacting theporous silicon deposition surface with a reactive gas mixture comprisingat least one chemical species comprising a transition metal and at leastone silicon chemical species; and depositing a silicon silicide layer onthe porous silicon surface.

Crystalline silicon compound layers and substrates of the presentinvention can have less dislocation density compared to direct growth onSi substrate due to accommodation of strain in the porous layer.Moreover, silicon compound layers of the present invention can achieveany desired thickness by utilizing techniques such as chemical vapordeposition, metal organic chemical vapor deposition, or any other meansknown in the art. Depending on whether the desired endpoint of theprocess is a hybrid silicon/silicon compound integrated circuit or asilicon compound integrated circuit or component, the silicon seed layermay be removed after final deposition.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects will become more readily apparent byreferring to the following detailed description and the appendeddrawings in which:

FIG. 1 illustrates a flow chart according to one embodiment of a methodof the present invention.

FIG. 2 illustrates an example of an apparatus for forming poroussilicon.

FIG. 3 illustrates prior art of heteroepitaxy of SiC on Si.

FIG. 4 illustrates heteroepitaxial growth of SiC on porous Si accordingto an embodiment of the present the invention.

FIG. 5 illustrates a result of heteroepitaxial growth of SiC on porousSi according to an embodiment of the present invention.

FIG. 6( a) displays preliminary cross-sectional scanning electronmicroscopy results of a silicon carbide layer produced according to anembodiment of a method of the present invention.

FIG. 6( b) displays x-ray diffraction results of a silicon carbide layerproduced according to an embodiment of a method of the presentinvention.

DETAILED DESCRIPTION

The present invention comprises methods of manufacturing siliconcompound crystalline layers and substrates. In one embodiment, a methodfor the production crystalline silicon compound layer comprises heatinga porous silicon deposition surface of a porous silicon substrate to atemperature operable for epitaxial deposition of at least one atom ormolecule; contacting the porous silicon deposition surface with areactive gas mixture comprising at least one chemical species comprisinga group IV element and at least one silicon chemical species; anddepositing a silicon-group IV element layer on the porous silicondeposition surface.

In another embodiment of the present invention, a method for theproduction of a crystalline silicon compound substrate comprises heatinga porous silicon deposition surface of a porous silicon substrate to atemperature operable for epitaxial deposition of at least one atom ormolecule; contacting the porous silicon deposition surface with areactive gas mixture comprising at least one chemical species comprisinga group IV element and at least one silicon chemical species; depositinga silicon-group IV element layer on the porous silicon depositionsurface; and separating the silicon-group IV element layer from theporous silicon substrate.

In some embodiments, once the silicon-group IV element layer isseparated from the porous silicon substrate, further growth of thesilicon-group IV element layer can be sustained. Further growth can besustained by continued epitaxial growth comprising heating thesilicon-group IV element layer, contacting the layer with a reactive gasmixture comprising at least one chemical species comprising a group IVelement and at least one silicon chemical species, and depositing asilicon-group IV compound on the heated layer.

In embodiments of the present invention, group IV elements correspond tothose of group IV of the periodic table comprising carbon, silicon,germanium, tin, and lead.

In another embodiment of the present invention, a method for theproduction of a silicon silicide layer comprises heating a poroussilicon deposition surface of a porous silicon substrate to atemperature operable for epitaxial deposition of at least one atom ormolecule; contacting the porous silicon deposition surface with areactive gas mixture comprising at least one chemical species comprisinga transition metal and at least one silicon chemical species; anddepositing a silicon silicide layer on the porous silicon surface.

In some embodiments, the present method can further comprise processingthe deposited silicon silicide layer to form silicon silicidesemiconductor devices. In other embodiments, the deposited siliconsilicide layer may be removed from the porous silicon substrate andsubsequently processed to form silicon silicide semiconductor devices.In some embodiments where the silicon silicide layer is removed from theporous silicon substrate, further epitaxial growth of the siliconsilicide layer can continue by heating the formed silicon silicide layerto a temperature suitable for epitaxial growth, contacting the siliconsilicide layer with a reactive gas mixture comprising a chemical speciescomprising a transition metal, and depositing a silicon silicide on theheated layer.

In some embodiments of methods of the present invention for theproduction of silicon silicide crystalline layers, transition metals cancomprise titanium, chromium, iron, cobalt, nickel, palladium, platinum,and/or molybdenum. In other embodiments, any other transition metalknown to one of ordinary skill in the art as suitable for the formationof silicon silicides can be employed.

In some embodiments of the present invention, the porous silicondeposition surface can be heated to a temperature operable for epitaxialdeposition of at least one atom or molecule wherein the temperature ofthe deposition surface can range from about 100° C. to about 1400° C. orgreater. In some embodiments of the present invention, the poroussilicon deposition surface can be heated to a temperature of rangingfrom about 100° C. to about 500° C. In other embodiments of the presentinvention, the porous silicon deposition surface can be heated to atemperature ranging from about 200° C. to about 900° C. In otherembodiment of the present invention, the porous silicon depositionsurface can be heated to a temperature ranging from about 900° C. toabout 1400° C.

In some embodiments where epitaxial growth of the formed siliconcompound layer is continued after removal of the layer from the poroussilicon substrate, the silicon compound layer can be heated up to about2700° C. to continue the epitaxial growth.

In some embodiments of the present invention, a reactive gas mixture cancomprise trimethylsilanes, methylsilanes, dimethylsilanes, ethylsilanes,diethylsilanes, and/or mixtures thereof. In other embodiments, thereactive gas mixture can comprise SiH₄, SiCl₄, SiH₃Cl, CH₄, C₂H₆, GeH₄,digermane, alkyl germanes, such as ethyl and diethyl germane, SnH₄,SnCl₄, other metal organics, and/or mixtures thereof. In someembodiments direct deposition of the elements comprising carbon,silicon, germanium, and tin is achieved. In other embodiments, thereactive gas mixture can comprise chemical species comprising cobalt,chromium, platinum, palladium, nickel, iron, titanium, and/or molybdenumoperable for the production of silicides.

In some embodiments of the present invention, crystalline silicon-groupIV layers can be removed from the porous silicon substrate for furtherprocessing into various semiconductor devices. In other embodiments, thecrystalline silicon-group IV layers can remain deposited on the poroussilicon substrate for further processing into various semiconductordevices.

In some embodiments, methods of the present invention can producecommercial size silicon-group IV substrates, such as SiC, that arecompatible with existing semiconductor fabrication tools used in Sitechnology, enabling economic production of wide band gap semiconductordevices and integrated circuits. Wide band gap devices includeradiation-resistant devices, high-power, high-frequency devices, shortwavelength electro-optic devices (including blue to ultraviolet sensorsand emitters).

In other embodiments, methods of the present invention can produce andintegrate islands of silicon-group IV element circuitry on andinterconnected with conventional Si integrated circuits. For example, ashort wavelength light sensor may be incorporated with longer wavelengthsilicon light sensors and silicon integrated circuitry for theprocessing of the signals derived therefrom.

Embodiments of the present invention will now be illustrated in thefollowing non-limiting example.

Example 1 Preparation of a Silicon Carbide Crystalline Layer

Example 1 demonstrates the preparation of a silicon carbide (SiC)crystalline layer according to one embodiment of the present invention.Referring to FIG. 1, a method 10 produces either a SiC wafer endpoint 26or a SiC island in a Si environment (an hybrid endpoint) 18. Asubstantially conventional silicon 12 wafer is used as a staringmaterial. Depending on the endpoint, the wafer 12 may be processeddirectly in an anodization step 14, or, using conventionalphotolithography means to open discrete locations on the wafer, thesilicon wafer may be prepared for the hybrid endpoint 18. The wafer isthen anodized according to the method described in detail with regard toFIG. 2. The anodization produces a porous layer of the silicon thatconsists of a labyrinth of fissures, voids, hillocks, andmicroscopically rough surfaces. The depth of the voids can be from 10-nmto the full thickness of the wafer. The lateral density of the voidsdepends on the anodization conditions. The feedstock silicon waferpreferably has one of the conventional lattice orientations, [110],[100] or [111] while the labyrinth of fissures, voids, hillocks, andmicroscopically rough surfaces exposes every conceivable angularinclination internal to the labyrinth.

The silicon wafer with an anodized surface or islands of anodizedregions is introduced to an epitaxial reactor 16. The epitaxial reactoris preferably a chemical vapor deposition (CVD) reactor. The reactantscontain at least a carbon-bearing gas and preferably a gas that containscarbon and silicon. CVD can be carried out and a reaction within thelabyrinth of fissures, voids, hillocks, and microscopically roughsurfaces can take place. The reactant gases can be doped to containdonor or acceptor atoms to create an n-type or p-type endpoint. Theenergy for the reaction can be supplied by heat, plasma excitation oroptical excitation. The reaction temperature has two regimes, at highertemperatures the reaction is dominated by mass transfer and the filmgrowth rate is substantially temperature independent. At lowertemperatures the growth rate shows an Arrhenius behavior. Arrheniusbehavior means that the growth rate decreases linearly against 1/T,where T is the temperature in ° K. and the slope is proportional toE_(A), the activation energy. The specific temperature for the crossoverfrom the first to second regimes depends on the pressure, reactants andenergy source. The values cited hereinafter are typical and not intendedto limit the invention in any way.

A 3C—SiC single crystal thin-film is formed using CVD on the poroussilicon surface within the labyrinth of fissures, voids, hillocks, andmicroscopically rough surfaces formed by the anodization step. Forexample, the silicon substrate may be heated to held at a temperature inthe range of about 900° C. through about 1400° C. The silicon source maybe SiH₄, SiCl₄, (CH₃)₃SiCl, (CH₃)₂SiCl₂, CH₆S₁, C₂H₈Si or any othersilane gas. The latter four examples provide a single gas source whereboth Si and C are provided. The carbon source may be CCl₄ or ahydrocarbon gas (C₂H₂, C₂H₆, CH₄, C₃H₈, etc.) Hydrogen or argon may beused as a carrier gas. A partial pressure of dopant gases may be addedto the mix.

A 3C—SiC single crystal thin-film is formed using CVD on the siliconsurface within the labyrinth of fissures, voids, hillocks, andmicroscopically rough surfaces formed by the anodization step. The waferwith the porous surface is reacted with a carbon and silicon-bearing gassuch as trimethylsilane (shown) at a temperature from about 900° C. toabout 1400° C. to form a SiC layer of any desired thickness. Because thereaction surface consists of a labyrinth of fissures, voids, hillocks,and microscopically rough surfaces formed by the anodization step,conversion to SiC and growth of SiC layer start within the porous layer.The resultant film is substantially stress free because the voids in theporous layer permit alignment of the two crystalline lattices by skippedbonds. If desired, the endpoint film may be doped to form n-type orp-type SiC by the introduction of a partial pressure of gases containingthe appropriate dopants. Other suitable carbon and silicon-bearing gasesinclude but are not limited to methylsilane, dimethylsilane,ethylsilane, and diethylsilane.

A 3C—SiC single crystal thin-film is formed using CVD on the siliconsurface within the labyrinth of fissures, voids, hillocks, andmicroscopically rough surfaces formed by the anodization step. Carbonatoms may be derived from graphite or hydrocarbon due to the thermaldecomposition and diffused into the labyrinthine surfaces on the siliconsubstrate surface.

In methods of the present invention, the porous Si with its labyrinth offissures, voids, hillocks, and microscopically rough surfaces formed bythe anodization step, acts as an elastic seed for the crystalline growthof the lattice-mismatched material. The labyrinthine structureaccommodates the lattice-mismatch virtually minimizing misfitdislocations. This is critical in the development of subsequent devices.For example, high dislocation density implies short carrier lifetimesand thus unsatisfactory circuit performance.

The thickness of the resultant, substantially stoichiometric SiC filmcan be quite large. It can be made to any desirable thickness byadjusting the growth parameters.

In one embodiment, the endpoint is silicon/silicon carbide implyingfurther processing to form a hybrid integrated circuit. In this case,the SiC is formed in islands in a conventional Si wafer. Furtherprocessing includes the formation of SiC devices and circuitsoperatively interconnected to and integrated with conventional Sicircuitry. For example, the short wavelength associated with the wideenergy gap of SiC enables blue to ultraviolet light sensors andemitters. Said sensors or emitters may desirably be interconnected withSi integrated circuitry enabling a new range of applications, forexample short wavelength fiber-optic communications.

In another embodiment, the endpoint is a SiC substrate. In this case,the entire surface of the silicon wafer feedstock is anodized andrebuilt into a thick (>20 μm) SiC layer and the Silicon is removed byany convenient means, for example mechanically, chemically or thermally.The resultant SiC wafer can be processed to a final thickness in asecond epitaxial step yielding a SiC endpoint. Further processing canresult in SiC devices or integrated circuits. This low cost SiCsubstrate can be processed in conventional ways to enable unconventionalresults that are implied by the large band gap. For example,radiation-resistant integrated circuits are readily fabricated. Furtherprocessing may include any of the steps encountered in siliconprocessing for diffusion of donor or acceptor species. For example, SiCspecific structures may use ion implantation.

FIG. 2 elaborates the principles of the anodization step 14. Theanodization produces a porous layer of the silicon that consists of alabyrinth of fissures, voids, hillocks, and microscopically roughsurfaces. The anodization may be accomplished in a container 30 that isfabricated from or totally lined with a material that is inert to theelectrolyte which is preferably hydrofluoric acid and ethanol. Forexample, the material may be a tetrafluoroethylene polymer such asTeflon© which is well known as a very chemically inert material over awide temperature range (−80° C. to 250° C.). The wafer 34 is clampedagainst a Teflon block 38 and electrically contacted with a tungstenclamp 32 to become the anode. The current density is linearly related tothe current since the geometry of the apparatus is fixed. The ammeter isin series with a power supply 42. The negative terminal of the powersupply is operatively connected to a tungsten cathode of fixed geometry.Tungsten was chosen because it is extremely stable in an acidicenvironment such as the HF containing electrolyte. The figure and thedescription hereabove illustrate the principles of the anodizationapparatus. The principles may be embodied in a variety of manufacturingtooling which may or may not superficially resemble the illustration inFIG. 2.

Referring to FIG. 3, the effect of heteroepitaxy on silicon according tothe prior art is illustrated. The silicon crystal 52 is represented byits lattice points with the lattice constant for silicon represented as“a” 58. The epitaxial layer of SiC 54 is grown on the silicon base. Thelattice constant for silicon carbide is represented as “b” 56. Theinterface 62 between the silicon and the silicon carbide layersillustrate stress bonds as bond on an angle. This lattice constantdifference induces many dislocations at the interface 62.

These dislocations exert an adverse effect on the electronic propertiesof the silicon carbide single crystals obtained and may trigger theformation of stacking faults in the crystal, thereby making it difficultto obtain silicon carbide semiconductor devices with excellentcharacteristics. Moreover, silicon carbide single crystals have atendency to contain crystal defects referred to as antiphase boundaries,thereby making it difficult to produce silicon carbide semiconductordevices at desired positions on a silicon substrate. Previous attemptsat epitaxial growth of SiC on Si have met with limited success. Forexample, minority carrier lifetimes have been impractically short. Themaximum obtainable thickness of the resultant films have beenimpractically thin for the separation of a stand-alone SiC wafer.

FIG. 4 illustrates the silicon substrate 70 by its lattice pointsaccording to this invention. The surface of the silicon where siliconcarbide is desired has been anodized leaving porous Si with itslabyrinth of fissures, voids, hillocks and microscopically roughsurfaces. As in FIG. 3, the lattice constant is represented by “a” 58.In the epitaxial reactor, a silicon and carbon-bearing gas such astrimethylsilane 74 reacts with the silicon substrate as in FIG. 3.However, the surface of the silicon confronting the reactive gas hasbeen prepared with multiple voids 72 a, 72 b and 72 c. The voids allowthe reacting gas to penetrate the confronting surface. The irregularinternal surface permits the SiC reactants to penetrate into the porouslayer and react laterally to form SiC while at the same timeaccommodating the resulting strain due to the lattice mismatch. Thesmall dimension of Si in the porous layer and presence of voids providethe mechanism by which strain is accommodated.

Moreover, the SiC material can bridge voids and continue to grow to forma continuous layer. A feature of CVD is that the reactant gas canpenetrate the labyrinthine surface in such a way that the reaction takesplace on the internal surfaces. As the reaction builds through thelabyrinth, the voids partially fill and are finally bridge withsubstantially strain-relieved SiC. It should be noted that contaminationcan be a source of poor results in CVD reactors. The wafers are placedin the reactor vessel, the reactant gases are introduced with a carriergas that is inert in the reaction. As the reactants are depleted, theirpartial pressure must be maintained by providing make-up gas as thereaction proceeds. In some CVD reactors, the gas may become contaminateddue to the inadvertent introduction of materials from the inside wallsof the vessel. Known ways to ameliorate said contamination problems suchas “cold wall” CVD may be applied here. The term “cold wall” refers tothe common case wherein the wafer and its support are heated whilekeeping the container walls cold. The advantage is that the walls do notsubstantially evolve contaminating materials during the film growth.

FIG. 5 shows the effect of a porous silicon substrate 70 confronting theepitaxial SiC layer. The pores permit stress-relieving gaps 72 a, 72 b,72 c, and 72 d in the bonds allowing the bonds to restart at minimumenergy pinning points. As a result, the epitaxial film 76 issubstantially free of the strain and defects such as dislocations andstacking faults described under FIG. 3. Note that the bonds at theinterface 78 are now substantially relaxed.

FIG. 6 shows specific results from a specific realization of thisinvention using scanning electron microscopy (SEM) and X-raydiffraction. These results do not represent a final process. However,important conclusions related to this invention can be made out of thefigures. The SEM image shows a cross-sectional image of SiC on Si. Inthis case, the growth process parameters were tuned to lead to selfseparation of the SiC layer from the Si (Note the gap between the twolayers). The SiC layer can be easily separated by twisting the waferwith respect to the layer. The X-ray results shows only one orientationSi(100). This is demonstrated by the peaks from the two equivalentSiC(200) and SiC(400) orientations that are parallel to the [100]direction. There are no measurable traces of any other orientation. Thisis a clear indication that the SiC is single crystalline layer grownalong the [100] direction.

A 3C-silicon carbide layer produced by one embodiment of methods of thepresent invention can avoid problems that result from the fact thelattice constant of silicon single crystals is different from that ofsilicon carbide single crystals by as much as 20%. In unimprovedmethods, the lattice constant difference induces a high density ofmisfit dislocations as well as stacking faults generated on the [111]planes within the silicon carbide single crystals grown on the siliconsingle-crystal substrate. Both misfit dislocations and stacking faultsexert an adverse effect on the electronic properties of the siliconcarbide single crystals obtained, thereby making it difficult to obtainsilicon carbide semiconductor devices with desired and reproduciblecharacteristics.

In some embodiments of the present invention, the crystalline siliconcarbide layers can comprise cubic silicon carbide. In other embodimentsof the present invention, the crystalline silicon carbide layers cancomprise hexagonal silicon carbide.

Additionally methods of the present invention can produce commercialsize silicon carbide substrates that are compatible with existingsemiconductor tooling, enabling economic production of wide band gapsemiconductor devices and integrated circuits that includeradiation-resistant devices, high-power, high frequency devices, shortwavelength electro-optic devices (including blue to ultraviolet sensorsand emitters); moreover, the present invention can provide methods tointegrate islands of SiC circuitry on and interconnected withconventional Si integrated circuits.

It is to be understood that the foregoing description and specificembodiments are merely illustrative of the best mode of the inventionand the principles thereof, and that various modifications and additionsmay be made to the apparatus by those skilled in the art, withoutdeparting from the spirit and scope of this invention, which istherefore understood to be limited only by the scope of the appendedclaims.

1. A method for the production of a silicon silicide layer comprising:heating a porous silicon deposition surface of a porous siliconsubstrate to a temperature operable for epitaxial deposition of at leastone atom or molecule; contacting the porous silicon deposition surfacewith a reactive gas mixture comprising at least one chemical speciescomprising a transition metal and at least one silicon chemical species;and depositing a silicon silicide layer on the porous silicon depositionsurface.
 2. The method of claim 1, wherein the transition metalcomprises titanium, chromium, iron, cobalt, nickel, palladium, platinum,or molybdenum.
 3. The method of claim 1, further comprising separatingthe silicon silicide layer from the porous silicon substrate.
 4. Themethod of claim 3, further comprising processing the silicon silicidelayer to form silicon silicide semiconductor devices.
 5. The method ofclaim 1, further comprising processing the silicon silicide layer toform silicon silicide semiconductor devices.